1. Field of the Invention
The present invention relates to a reflection type ion attachment mass spectrometry apparatus, and more particularly, relates to a reflection type ion attachment mass spectrometry apparatus for measuring ingredients of a measured gas with a high sensitivity and high precision, and a method of reflection type ion attachment mass spectrometry.
2. Description of the Related Art
Ion attachment mass spectrometry (IAMS) is a method of ionizing the molecules of a measured gas without causing fragmentation (causing the generation of fragments, that is, breaking up the original molecules), making the ions of the molecules move to the mass spectrometry region, and analyzing their mass there. There are the following documents concerning apparatuses for working the ion attachment mass spectrometry method in the related art.
As patent documents, there are JP-A-6-11485, JP-A-2001-174437, JP-A-2001-351567, JP-A-2001-351568, JP-A-2002-124208, JP-A-2002-170518, U.S. Pat. No. 4,933,551, and U.S. Pat. No. 4,649,278. Further, as other documents, there are (1) Hodge, xe2x80x9cAnalytical Chemistryxe2x80x9d, 1976, vol. 48, no. 6, p. 825, (2) Bombick, xe2x80x9cAnalytical Chemistryxe2x80x9d, 1984, vol. 56, no. 3, p. 396, (3) Fujii, xe2x80x9cAnalytical Chemistryxe2x80x9d, 1986, vol. 61, no. 9, p. 1026, and (4) Fujii, xe2x80x9cChemical Physics Lettersxe2x80x9d, 1992, vol. 191, no. 1.2, p. 162.
Referring to FIG. 22, the general configuration of an apparatus for ion attachment mass spectrometry of the related art will be explained in relation to the present invention. In FIG. 22, 1 indicates a metal ion generation region, 2 an attachment region, and 3 a mass spectrometry region. The metal ion generation region 1 and attachment region 2 form a common compartment having a common vacuum environment. A partition 6 is provided between the attachment region 2 and mass spectrometry region 3. The partition 6 is formed with an aperture 6a. The metal ion generation region 1 is provided with a metal ion emitter 4. In FIG. 22, 5 shows the path of movement of the metal ions and attached ions. The mass spectrometry region 3 is provided with a mass spectrometer 8 and is additionally provided with a vacuum pump 7.
The metal ion generation region 1, the attachment region 2, and the mass spectrometry region 3 all are at reduced pressures of not more than atmospheric pressure. In the metal ion generation region 1, a metal ion emitter 4 of an oxide of an alkali metal is heated to generate Li+ and other positively charged metal ions. The metal ion emitter 4 is heated by supplying current by application of voltage by a not shown power source. The metal ions are transported to the attachment region 2 from the metal ion generation region 1 by an electric field. The measured gas (or sample gas) is introduced into the attachment region 2 by a measured gas introduction mechanism 30. The metal ions gently attach to locations with a concentration of charges of molecules of the measured gas. The molecules to which the metal ions are attached become positively charged ions as a whole, whereby attached ions (pseudo molecular ions) are generated.
At the time of attachment, the surplus energy is extremely small, so fragmentation does not occur. However, to prevent the metal ions from again disassociating from the attached ions (prevent the metal ions from detaching from the molecules of the measured gas), it is necessary to strip the surplus energy by having the ions collide with the ambient gas. To raise the efficiency of attachment, it is necessary to decelerate the metal ions emitted from the metal ion emitter 4 by the high voltage down to a translational energy of not more than 1 eV by colliding with the ambient gas. Even if the metal ions having a translational energy of at least 1 eV contact the molecules of the measured gas, almost all of them end up separating without attachment. To maximize these two effects, the general practice in an ion attachment mass spectrometry apparatus of the related art is to make the pressure in the attachment region 2 about 100 Pa. With a pressure of 100 Pa, the movement of the ions is not smooth and a problem arises in the quantitativeness of the results of measurement. Therefore, recently, methods of deceleration by an electric field and operation at a pressure of about 1 Pa in the attachment region are being developed.
The attached ions produced as explained above are again accelerated by the electric field, passed through the partition 6 with the aperture 6a, and transported to the mass spectrometry region 3. A Q-pole type mass spectrometer or other mass spectrometer 8 using electromagnetic force measures the attached ions separated in mass-charge ratio (mass number). The mass spectrometer normally can only operate by at a pressure of not more than 10xe2x88x923 Pa, so a pressure difference is generated by the partition 6 with the aperture 6a. FIG. 22 shows a general example of the related art, but in different related arts, the differential regions exist in some cases, and do not exist in the other cases, and the number of vacuum pumps, etc. differ.
Therefore, in the past, an electron attachment mass spectrometry apparatus has also been proposed (U.S. Pat. No. 4,933,551). According to the electron attachment mass spectrometry apparatus disclosed in the above-mentioned document, electrons are made to attach to the neutral gas to form negative ions as a whole for mass spectrometry. Further, the technical idea has been proposed of slowing the speed of the electrons using an electric field and causing the electrons to attach to the gas molecules to create negative ions (U.S. Pat. No. 4,649,278). According to this document, a mirror electrostatic field is used to make the speed of the electrons zero or nearly zero and enable electrons to be attached to the gas.
The ion attachment mass spectrometry apparatuses of the related art all could ionize molecules without causing fragmentation and could correctly identify the ingredients of the measured gas (quantitative analysis). This surpasses other techniques. The scientific and industrial fields have large expectations vis-a-vis ion attachment mass spectrometry apparatuses. However, ion attachment mass spectrometry apparatuses have the weak point that the measurement sensitivity is insufficient and detection of trace ingredients is difficult. In particular, with the method of making the pressure of the attachment region 2 1 Pa for the purpose of improving the quantitativeness, the measurement sensitivity ends up deteriorating more, so the insufficient measurement sensitivity becomes a serious problem.
The reasons for the insufficient sensitivity will be explained in the following. As shown in FIG. 22, in the ion attachment mass spectrometry apparatus of the related art, the metal ion generation region 1, attachment region 2, and mass spectrometry region 3 are positioned adjoining each other in that order, while the metal ion emitter 4 and the mass spectrometer 8 are arranged on substantially the same straight line straddling the attachment region 2. Therefore, even if the metal ions generated from the metal ion emitter 4 change to attached ions in the attachment region 2, they are not changed in direction and proceed straight to the mass spectrometer 8 as they are. If there is no change in the translational energy on the way, the system of the conventional art of making the ions proceed straight is easiest and most reliable in control of the ions. However, in fact, the ions are greatly decelerated and accelerated in the process. Sufficient control has not been possible in the conventional art.
Specifically, for extracting and transporting the metal ions from the metal ion emitter 4, first, the translational energy of the metal ions is 10 to 20 eV, but at the attachment region 2, the ions are decelerated to less than 1 eV to improve the attachment efficiency. Next, the attached ions produced are accelerated and again transported to the mass spectrometer 8 by a translational energy of 10 to 20 eV. If decelerated sharply in this way, the horizontal direction translational energy component originally held by the individual ions is strongly manifested and the ions end up spreading broadly spatially. It is extremely difficult to again accelerate and concentrate the ions spreaded out spatially in the substantially stopped state. Therefore, the actually detected attached ions become an order of magnitude smaller than the total amount of the attached ions produced.
In particular, when improving the quantitativeness by deceleration by only an electrostatic field without using collision with a gas, this problem becomes more serious. Only with an electrostatic field, decelerating and stopping the ions, then accelerating them again in the same direction is inherently impossible. This becomes possible only by using an electric field changing along with time, but the process of transport and attachment becomes intermittent, so the time averaged efficiency drops sharply. For example, in U.S. Pat. No. 4,649,278, an electric field changing over time is used.
As explained above, the problem of ion control in deceleration and acceleration while advancing is a major reason for insufficient measurement sensitivity in an ion attachment mass spectrometry apparatus of the conventional art.
In an ion attachment mass spectrometry apparatus of the conventional art where the metal ion emitter 4 and the mass spectrometer 8 are arranged facing each other on the same line, there were the following problems (1) to (4) in addition to the above problem of insufficient measurement sensitivity.
(1) The mass spectrometer 8 is easily disturbed by light or neutral particles etc. from the metal ion emitter 4. (2) The metal ion emitter 4 easily deteriorates. (3) The apparatus is large in size. (4) Direct monitoring is difficult.
The above problems will be explained in more detail. Light, neutral particles, etc. are also produced from the metal ion emitter 4 heated to a high temperature, but these do not carry a charge, so they proceed directly, enter the facing mass spectrometer 8, and cause a rise in the background level, contamination of the electrodes, etc. In particular, a general Q-pole type mass spectrometer used as the mass spectrometer 8 is comprised of four axially parallel poles, so the light or neutral particles proceeding directly from the surface, that is, flying in parallel to the axis, enter deep into the Q-pole type mass spectrometer and cause an extremely serious problem.
The metal ion emitter 4 is positioned at spatially the same region as the attachment region 2 to which the measured gas is introduced. Therefore, the surface of the metal ion emitter heated to a high temperature is continuously exposed to the measured gas, the measured gas reacts at the surface of the metal ion emitter, the products deposit on the surface of the metal ion emitter, and the surface is etched and deterioration of the metal ion emitter 4 is caused.
Further, as the structure of the apparatus, the metal ion generation region 1, attachment region 2, and mass spectrometry region 3 are aligned in that order, so the apparatus becomes larger as a whole. The attachment region 2 in which the measured gas is introduced is positioned at the center of the apparatus, so direct sampling where the measured part is connected directly to the attachment region without using piping which might cause a change in the ingredients of the measured gas, is difficult to implement.
Further, in the electron attachment mass spectrometry method disclosed in U.S. Pat. No. 4,933,551, steps to decelerate the electron are adopted. However, the electron attachment mass spectrometry method has the characteristic that the electrons attach to only the negative gas which easily becomes negative ions and in almost all cases fragmentation occurs after electron attachment. In the ion attachment mass spectrometry method, there is no such characteristic. This is a completely different ionization method. Further, with electron attachment, the electrons to be attached penetrate to electron orbit of the gas, while with ion attachment, the metal ions make a gentle bonding with the gas. Due to the above, the electron attachment mass spectrometry method is an extremely special method of analysis only for some scientific research, while the ion attachment mass spectrometry method can be considered as an extremely general method of analysis for use in a broad spectrum of industries.
From another viewpoint, in the ion attachment mass spectrometry apparatus of the related art shown in FIG. 22, the pressure of the metal ion generation region and the pressure of the attachment region become substantially the same and the problem arises that the metal ions emitted from the metal ion emitter become harder to decelerate. Therefore, it is desirable to configure the apparatus so as to enable a sufficient difference to be given between the pressure of the metal ion generation region and the pressure of the attachment region and to enable the metal ions emitted from the metal ion emitter to be sufficiently decelerated on the way toward the attachment region.
An object of the present invention is to provide a reflection type ion attachment mass spectrometry apparatus which can detect trace ingredients with a high measurement sensitivity, can solve or minimize the problems of disturbance of the mass spectrometer, deterioration of the metal ion emitter, the size of the apparatus, and the direct sampling, and can be widely used in industry as a general analytical method.
Another object of the present invention is to provide a reflection type ion attachment mass spectrometry apparatus which can make the pressure of the metal ion generation region and the pressure of the attachment region different so as to sufficiently decelerate the metal ions.
The reflection type ion attachment mass spectrometry apparatus according to embodiments of the present invention is configured as follows for achieving the above object.
A first reflection type ion attachment mass spectrometry apparatus is an apparatus causing positively charged metal ions generated in a metal ion generation region to attach to molecules of a measured gas in an attachment region to generate attached ions and then performing mass spectrometry on the attached ions in a mass spectrometry region, wherein the metal ion generation region and the mass spectrometry region are formed as a common region or compartment, the attachment region is provided adjoining the common region, and the attachment region is provided with an electrostatic field generating means for forming an electrostatic field for reflecting the metal ions introduced from the metal ion generation region to the attachment region so as to guide them to the mass spectrometry region.
Preferably, a translational energy of the metal ions is reduced by reflection.
More preferably, the attached ions are accelerated and concentrated by the electrostatic field reflecting the metal ions.
More preferably, the electrostatic field generating means forms an electrostatic field without using a grid where the metal ions or the attached ions pass.
Preferably, a correction field is superposed on the electrostatic field to adjust the paths of the metal ions and attached ions.
Preferably, the distribution of intensity of the electrostatic field is of axially symmetric ellipsoid.
Alternatively, the distribution of intensity of the electrostatic field is spherical.
Preferably, a partition having an aperture for introducing the metal ions from the metal ion generation region to the attachment region and an aperture for transporting the attached ions from the attachment region to the mass spectrometry region is provided between the attachment region and the region of the metal ion generation region and the mass spectrometry region.
Alternatively, a partition having an aperture for introducing the metal ions from the metal ion generation region to the attachment region and transporting the attached ions from the attachment region to the mass spectrometry region is provided between the attachment region and the region of the metal ion generation region and the mass spectrometry region.
Here, the reflection type ion attachment mass spectrometry apparatus according to an embodiment of the present invention explained above will be explained with respect to its actions. The problems explained in the section on the related art are due to the fact that the metal ion emitter and mass spectrometer are positioned on substantially the same straight line and the metal ions and attached ions proceed straight. On the contrary, in the embodiments of the present invention, the metal ion emitter and mass spectrometer are arranged in the same compartment. Further, the metal ions are reflected in the attachment region to generate attached ions in the process of reflection, and the returning attached ions are concentrated and led to the mass spectrometer. Due to this, it is possible to find a basic solution to the above problems.
The characteristic behavior in an electrostatic field is that ions having the same translational energy fly over completely the same path without regard to the mass. This can be explained as follows. Heavy ions are slower in actual speed compared with light ions even with the same translational energy. Therefore, the time of passage through the electrostatic field becomes longer and the impulse received from the electrostatic field becomes stronger. However, since the mass is heavier, the acceleration (=force/mass) finally becomes the same. The path becomes completely the same as that of light ions. In the electrostatic field, the metal ions become attached ions midway and the mass of the ions increases, but there is no change in the path at all.
In the configurations of the present invention, the above problems are solved based on the actions and effects (a) to (g) explained below:
(a) When reflecting metal ions at an acute angle by an electrostatic field, since the metal ions are sufficiently decelerated, the attachment efficiency becomes extremely high.
(b) Since the attached ions generated at the time of deceleration are accelerated in the opposite direction, ion control of good precision becomes possible.
(c) Since, by suitably shaping the electrostatic field, even if the metal ions spread spatially, the attached ions will be concentrated over the reverse path, so high efficiency detection becomes possible.
(d) Since the metal ion emitter and the mass spectrometer do not face each other, the light or neutral particles from the metal ion emitter cannot enter the mass spectrometer and normal mass spectrometry becomes possible at all times.
(e) Since the metal ion emitter is the same compartment as the mass spectrometer which should be at not more than 10xe2x88x923 Pa, the contact with the measured gas is greatly reduced and there is no longer deterioration of the metal ion emitter.
(f) Since there is no longer an independent metal ion generation region, the apparatus can be made smaller in size.
(g) Since the attachment region is positioned at the front end of the apparatus, direct sampling by directly connecting the measured part is possible.
Next, a second reflection type ion attachment mass spectrometry apparatus is an apparatus causing positively charged metal ions generated in a metal ion generation region to attach to molecules of a measured gas in an attachment region to generate attached ions and then performing mass spectrometry of the attached ions in a mass spectrometry region, having a reflection type structural member introducing the metal ions to the attachment region, causing the metal ions to attach to molecules of measured gas to generate attached ions while reflecting the metal ions at the attachment region, and ejecting the attached ions from the attachment region, and performing mass spectrometry on the attached ions by a mass spectrometer, an aperture by which the metal ions enter the attachment region and an aperture by which the attached ions are ejected from the attachment region being the same common aperture.
Preferably, a path of the metal ions before entering the common aperture and a path of the attached ions after departing from the common aperture are separated by an electric field or magnetic field.
Preferably, the distribution of the electric field of the attachment region is made spherically symmetric with respect to the common aperture.
Preferably, a supersonic jet is formed in the attachment region.
The reflection type ion attachment mass spectrometry apparatus of the present invention described above has the following action. In such low densities of the metal ions and attached ions as those in an ion attachment mass spectrometry apparatus, they have negligible effect on each other even if they pass by at the same position. Therefore, the structure of a reflection type ion attachment mass spectrometry apparatus is employed, the attachment region, metal ion generation region, and mass spectrometry region are arranged in that order, and the metal ions and attached ions are made to pass through a single same aperture (two-ion passing aperture) at the boundary of the attachment region and metal ion generation region.
In the metal ion generation region, the paths of the metal ions and attached ions are separated by the electric field or the magnetic field and the metal ion emitter and mass spectrometer are prevented from interfering with each other. In the attachment region, due to the use of a spherical electric field, the metal ions ejected from the two-ion passing aperture return to the same location as the attached ions.
Further, as a alternative structure of the attachment region, a two-ion passing aperture of a skimmer type is inserted into the Mach disk inside which the supersonic flow is generated as in the same way as the supersonic type ion attachment mass spectrometry apparatus.
In the ion attachment mass spectrometry apparatus having a reflection structure, the metal ions can be sufficiently decelerated, the efficiency of generation of the attached ions can be raised, and the measurement sensitivity can be increased. By employing the structure of a two-ion passing aperture, it becomes possible to give a sufficient pressure difference. This is effective for measurement at a high pressure. On the contrary to this, in the case of a two-aperture structure, this becomes effective at a low pressure due to not disturbing the flight of the ions.
According to the first aspect of the present invention explained above, the ion attachment mass spectrometry apparatus forms the metal ion generation region and mass spectrometry region as a common region or compartment, provides the attachment region adjoining the common compartment, and is provided with an electrostatic generating unit for guiding the metal ions to the mass spectrometry region by reflection, so it is possible to detect trace ingredients by a high measurement sensitivity, and to solve the problems of disturbance of the mass spectrometer, deterioration of the metal ion emitter, apparatus size, direct sampling, etc., and realize broad applications in industry. According to the second aspect of the invention, in addition to the above advantageous effects of a reflection type ion attachment mass spectrometry apparatus, using a single aperture as a two-ion passing aperture is particularly effective for measurement of a sample gas with a high gas pressure. It is possible to make the pressure of the metal ion generation region and pressure of the attachment region different and sufficiently decelerate the metal ions.